This invention relates to circuits used to monitor the electrical signals produced by electrical power circuits and equipment, and more particularly to a novel approach to monitoring ac frequency signals over a wide frequency range.
There are presently a number of circuits used to monitor the frequency of a variable output wave and to detect frequency error. However, most of these circuits produce a signal for a frequency meter and occasionally, an error signal. None of the presently known methods produce frequency "go/no-go" signals which can be used to indicate whether the monitored frequency is within a specified band or out of the band. A "go/no-go" output signal would be highly valued by industry which can use it in many ways including the correction of the monitored frequency.
All the currently available methods have several drawbacks, including inability to respond to transient frequency changes, a relatively high component count producing low reliability, and a large board area resulting in high cost. In addition, these methods often have lapses in accuracy when monitoring high frequency waves, particularly when harmonics are present.
Thus there exists a need for a frequency monitor circuit that will produce a "go/no-go" signal output; that will be able to respond to transient frequencies while at the same being highly reliable and low in cost.
The present invention frequency monitor and error detector circuit includes a highly accurate crystal oscillator to generate a reference frequency, a binary counter, and an exclusive OR gate to determine the amount the input monitored frequency varies from the reference. An adjustable delay circuit is provided to check whether the input frequency is within a selected tolerance band. One-shots, a flip-flop and three AND gates are arranged to provide the logic means for output signals indicating whether the monitored frequency is within the tolerance band or out of the band. The circuit thus applies a continuous, simple "go/no-go" test to the monitored frequency while also being capable of responding to transient frequency change. The circuit components are small in size and few in number, making for low cost and high reliability.
Accordingly, it is a principal object of this invention to provide a frequency monitor and error detector circuit that produces a "go/no-go" frequency signal and is accurate at from low to high frequencies over a wide range.
Another object is to provide a frequency monitor and error detector circuit that is able to respond to transient frequencies.
A further object is to provide a frequency monitor circuit that is high in reliability, low in cost and small in size.
Further objects and advantages of the invention will be apparent from studying the following portion of the specification, the claims and the attached drawings.